there are 2 options i guess,you will need Pokemon Ruby,Sapphire or Emerald and find Deoxys in birth island(if you solve some kind o puzzle)and migrate it.
2.Get the Diamond or Pearl event in GameStop and the Action Replay,then talk to a guy in any PokeMart,he will give you a Deoxys then trade it.
The REAL thing to do...,well i don't know but go here.
http://www.serebii.net/pokedex-dp/386.shtml
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You can't catch Deoxys in soul silver
If you have action replay you can get any Pokemon you want. This feature works only in Pokemon Heartgold , Soulsilver and Platinum.
First you have to catch Mewtwo and then head to Mt Moon at midnight on Monday. Somewhere deep in the cave is were the deoxys are.
Sorry to rain on your parade, you can't get it on soulsiver. sorry! :) -pokeman
it does not exist in soulsilver outside of hacks or trades.